A two-dimensional (2-D) time-dependent cosmic ray modulation model is used to calculate the
modulation of cosmic-ray protons and electrons for 11-and 22-year modulation cycles using a
compound approach to describe solar cycle related changes in the transport parameters. The
compound approach was developed by Ferreira and Potgieter (2004) and incorporates the concept
of propagation diffusion barriers, global changes in the magnetic field, time-dependent
gradient, curvature and current-sheet drifts, and other basic modulation mechanisms. By comparing
model results with 2.5 GV Ulysses observations, for both protons and electrons, it is
shown that the compound approach results in computed intensities on a global scale compatible
to observations. The model also computes the expected latitudinal dependence, as
measured by the Ulysses spacecraft, for both protons and electrons. This is especially highlighted
when computed intensities are compared to observations for the different fast latitude
scan (FLS) periods. For cosmic ray protons a significant latitude dependence was observed for
the first FLS period which corresponded to solar minimum conditions. For the second, which
corresponded to solar maximum, no latitude dependence was observed as was the case for the
third FLS period, which again corresponded to moderate to minimum solar activity. For the
electrons the opposite occurred with only an observable latitude dependence in intensities for
the third FLS period. It is shown that the model results in compatible intensities when compared
to observations for these periods. Due to the success of the compound approach, it is
also possible to compute charge-sign dependent modulation for 2.5 GV protons and electrons.
The electron to proton ratio is presented at Earth and along the Ulysses trajectory. Lastly, it is
also shown how the modulation amplitude between solar minimum and maximum depends
on rigidity. This is investigated by computing cosmic ray intensities for both protons and electrons,
not only at 2:5 GV, but also up to 7:5 GV. A refinement for the compound approach
at higher rigidities is proposed. / Thesis (M.Sc. (Space Physics))--North-West University, Potchefstroom Campus, 2011.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/4402 |
| Date | January 2010 |
| Creators | Magidimisha, Edwin |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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